When I tried this build for the first time I was surprised by how good Janna was at both killing and supporting. I focused more on the supporting element (but if I wanted more kills, I very easily could of got them) Whenever I came across an enemy, or a group her Q and W skill would impact masses amount of damage which contributed a lot in team fights, and even sometimes caused enemies to flee from her which is rare considering Janna is assumed to be an easy target to kill 1v1, or gank which isn't always the case

SUMMONERS SPELLS.
I recommend ignite and flash. Ignite simply because this can be the difference between a kill, or no kill. But use it wisely, especially if the enemy is escaping and you can quickly catch them with it to execute you the kill and without putting you in further danger before you retreat to safety. And flash because again, when used correctly it can save you from the worst situations potentially saving your life, but also to catch up with an opponent low on health to do further damage on them.

MASTERIES
As you can see I've put many points towards into offence for the extra AP damage, and the AD is just for that little boost combined with her shield (E) which gives bonus attack damage. But also some mana, and magic resist as Janna in 1v1 situations or even ganks cannot spam skills, so that extra resist could save her life in some situations.

ITEMS
Janna can get assist kills very easily so Meija's soulstealer is very useful as the assists will help it stack, heavily increasing your AP damage. For her boots you can either go with Lucidity for the cd reduction, or sorceress's for the magic penetration if you're determined to kill. Deathcap is obvious, the AP boost is huge which means masses of damage. Rod of ages because of the boosted AP, but also the mana and health which is crucial for her defence and survivability. And Lich blane for the extra movement, and attack speed which is a nice boost for both your survivability (escaping ganks/chasing down enemies), and your damage.

JANNA'S PASSIVE, SKILLS, AND MOBILITY.
Firstly her passive 'Tailwind' is probably one of the best passives in the game. It increases your allies movement speed by 3% and that 3% can be the difference between an ally escaping, or his/her death. And also when playing Janna I find that her mobility is one of her very best features. Her movement speed allows her to escape dangerous situations, as well as track enemies down and possibly stopping them from escaping. Tornado (Q) is great for cancelling enemies who are chasing you, knocking them into the air gives you that extra bit of time to escape, but it can also stop enemies who are running away. Zephyr (W) is great for slowing down enemies which again can be a life-changing situation. Either using it to slow down predators who are chasing you to help you escape, or slowing down your prey to stop them from escaping and executing a kill. Her shield (E) can be used to either increase yours or your allies attack damage, or to simply protect you or them from any incoming damage. So use this wisely in the situation you're in. And Monsoon is great for healing, escaping and changing game strategy's which will explained more so further down.

FARMING
Last hitting minions is crucial for ANY character when you're planning on either being strict AD, or AP. But lucky for Janna if farming early game isn't as successful as you hope, when her Tornado (Q) hits lv 3-4 if you leave it to charge for it's full duration before releasing it this can wipe out a group of minions which will score you a lot of gold. Also activating her shield (E) will give her or a chosen ally bonus attack damage which will make last hitting minions a lot easier.

COMBOING AS JANNA.
When going in for the kill with Janna I tend to use Zephyr (W) first, followed activating her shield (E) for that increased normal attack damage (every damage helps) and then her tornado (Q) which with their movement speed already slowed there isn't a very good chance of them dodging this, and also this lifts them into the air which you can then strike them with more normal attacks. Then once Zephyr has finished it's cd I will use it again and repeat the process. (If they're close to getting away e.g close to a tower, running towards their allies) You can always use ignite to finish them off. Also, if you have enough movement speed to get in front of them you can use her ultimate Monsoon (R) to push them towards the direction you just came from in order to score more damage and hopefully secure a kill.

SUPPORTING
Now as I previously stated this build still guarantee's Janna a healthy support system. With that increased damage from cancelling the enemy team with Q (also potentially saving your team-mates who are being chased) and slowing a single enemy down with W (which can also help your team mate get away from predators if they are slowed) you will be good help. Using her shield (E) on the strongest AD champion during ganks will increase their damage so hopefully they can execute kills more faster. But you can also use the shield to protect your allies who are losing health and potentially saving their life. And with monsoon this is pretty obvious, you can blow the team away from ganking an ally, separate them, push the enemy towards the direction you want them for a gank, or simply to regenerate some health. Use your ultimate wisely in different situations as this can tip the scale in your teams favour.